The present invention is directed to an arrangement for the continuous melting of granulated silicon in a melt reservoir that, based on the principle of communicating vessels, is in communication with a melt crucible for the continuous, horizontal drawing of silicon bands.
An arrangement, as well as, an apparatus and a method for the manufacture of band-shaped silicon crystals having horizontal drawing direction is disclosed in European Patent application 0 170 119.
Pursuant to this method (referred to as S-web method), and with this apparatus, silicon bands can be drawn from a silicon melt with a high drawing speed (about 1 meter per minute). In order to fully exploit, economically, the possibilities of the method, it is necessary to operate the drawing process continuously over several hours or days. A critical prerequisite therefor is that the surface of the silicon melt, in the crucible from which the silicon band is drawn, maintains a constant level during the entire duration of the drawing process.
The problem of maintaining a constant level, is that silicon raw material must be continuously supplied to the crucible without thereby disturbing the band drawing process. For example, the continuous delivery of about 350 g silicon per minute is required for a band that is 30 cm wide and 0.5 mm thick given a drawing speed of 1 meter per minute.
Silicon raw material is available in the form of granules. Since the density of solid silicon is lower than that of molten silicon, the granules float when scattered onto the melt, without being submersed therein. The heat transmission from the melt into the granules is therefore poor. This results in the accumulation of unmelted granules on the melt, under the filling location. The heat transmission within the accumulated granules is even poorer than the transmission between the melt and individual granules. In the known melt-down methods of the iron and steel industry, the delivered material sinks completely into the melt due to the density ratio of the solid to molten substance prevailing therein. This is opposite to that of silicon and the difficulties set forth above therefore do not occur in the iron and steel industry.
For a number of reasons, one can not increase the temperature of the silicon melt to such an extent above the melting temperature of silicon that adequate thermal output is transferred into the granules, due to a very high temperature gradient even given poor heat transmission One problem is that the quartz used to construct the melt and drawing crucible becomes increasingly mechanically unstable at temperatures above the melting point of silicon. With increasing temperature, the rapidity of the chemical reaction between quartz and molten silicon increases, this leads to silicon oxide being created that evaporates out of the melt given the high temperatures prevailing, and forms carbon monoxide with the highly heated graphite surfaces (the heaters and, covers). Carbon monoxide is in turn dissolved in the silicon melt and thus forms silicon carbide pursuant to the equation: EQU CO+2 Si.fwdarw.SiO+SiC
The resultant silicon carbide is incorporated into the silicon bands, and has an extremely deleterious effect on the silicon bands use for solar cells.